The present invention relates to a new and improved method and apparatus for the evaluation of signals from an ultrasonic displacement path or distance measuring system, especially for machine tools.
Generally speaking, the method and arrangement of the present development is of the type wherein there is provided a magnetostrictive line or conductor along which there can be displaced a measuring head containing permanent magnets and by means of which there are transmitted trigger pulses. These trigger pulses, in each case, set a flipflop and lead to echo pulses in the permanent magnets which reset the flipflop and are received by a stationary receiver of the measuring system. Within a signal transit or travel time, referred to herein as the transit time pulse, in other words, within the time between each trigger pulse and the received echo pulse there are counted, in a displacement path or distance counter, the pulses generated by a displacement path or distance oscillator as a measure for the distance or displacement path between the permanent magnets and a fixed reference point.
Such type of ultrasonic distance or displacement path measuring system is known, for instance, from U.S. Pat. No. 3,898,555, granted Aug. 5, 1975. With this distance measuring system an oscillator generates trigger pulses having a fixed frequency for the relevant magnetostrictive line or conductor and dependent upon its length, such frequency fluctuating between 8,000 Hz for short conductors or lines and 2,000 Hz for long conductors or lines. This fixed trigger pulse cycle is disadvantageous, because it fixes a predetermined measuring rate which cannot be enlarged. Also if the echo pulse appears briefly after the trigger pulse, because the measuring head is located at a slight distance from the receiver of the measuring system, then it is necessary to wait with the next trigger pulse by an amount of time governed by the clock frequency of the oscillator before there can be undertaken a new measurement. Moreover, the resolution with this state-of-the-art ultrasonic distance or displacement path measuring system is governed by the frequency of the distance or displacement path oscillator, since the greater the number of displacement path-oscillator pulses counted during a transit time pulse, that much greater is the resolution. If there is strived for an exceptionally high resolution, then there are quickly reached displacement path-oscillator frequencies which no longer can be processed. Finally, the prior art ultrasonic distance measuring system is only suitable for measuring the position of a stationary measuring head. If this measuring head were to move, then because of the finite propagation velocity of the ultrasonic echo pulse, amounting to approximately 2,800 m/s, there would be caused an error rendering impossible the use of the system with precision machine tools, for instance with large gear grinding machines.
Furthermore, there are known to the art from German Patent Publication No. 2,833,369, published Feb. 7, 1980, a method and arrangement for the incremental measuring of the relative position of two objects by means of ultrasonic waves, which afford a greater accuracy than the known ultrasonic distance measuring system in that, here, the phase between an alternating-current voltage, serving to generate the ultrasonic wave in the magnetostrictive conductor or line, and an alternating-current voltage induced in the receiver by the ultrasonic wave, is employed as a measure for the relative position of the receiver with respect to the transmitting measuring system. The evaluation technique is not dependent upon the amplitude of the transmitted trigger pulse and the received echo pulse. However, this method is afflicted with the shortcoming that there are not possible any absolute measurements.